The present invention generally pertains to the field of electronic circuits. More particularly, the present invention is related to a technique for detecting that a power supply is below a pre-determined level and generating an indicator signal.
In some applications such as the Real Time Clock for PC, it is necessary to detect the power supply level and generate an indicator signal when the power supply has dropped below a threshold level. This is because below the threshold level, circuit operations are not guaranteed.
One conventional method for a voltage level detection circuit uses bandgap reference comparison. Referring to FIG. 1, a reference voltage, Vref 106, the desired threshold level, is generated by bandgap reference 102 and is compared with the power supply 104 via differential amplifier 104. When the supply 104 drops below Vref 106, VX 108 will change. This circuit 100 can be accurate; however, it cannot be used as power on reset because Vref 106 is not available when the power is just switched on. Also, bandgap reference voltage level that is accurate over a wide temperature range centers only around 1.2 Volts. Therefore, a relatively limited range of threshold voltages may be detected. Furthermore, the bandgap reference increases the number of components in the voltage level detection circuit.
A second conventional method is a voltage divider configuration. Referring to FIG. 2, in this configuration, Z1 201 and Z2 202 are two impedances (e.g., resistors). They are chosen to have a ratio such that when VDD 204 is at a normal operating level VX 208 is low because transistor M1 206 is on. When VDD 204 drops to the selected threshold, Vi 210 will be lower than the threshold of M1 206. At that point, Vx 208 will switch to high because transistor M1 206 is off. This circuit 200 can be used as power on reset. However, since the threshold of M1 206 varies with temperature and process, the selected threshold of VDD 204 at which Vx 208 will change, therefore, also varies with temperature and process.
FIG. 3 illustrates how temperature and process variations may affect VGS versus IDS of a NMOS transistor. The graph 300 shows the variation to be over 400 mV between a high temperature (e.g., 125 degrees Celsius) fast corner device 302 and a low temperature (e.g., xe2x88x9240 degrees Celsius) slow corner device 308. Comparing the high temperature slow corner device 306 and the low temperature fast corner device 304 with the other curves reveals that even one of the variations (process or temperature) may account for approximately 200 mV differences in the device threshold. Moreover, in an actual circuit, the change in Vi could be as high as 1 Volt due to the amplification effect of the impedance ratio.
Thus, a need has arisen for a technique for detecting a power supply is at a low voltage level. A further need exists for such as system which also functions as a power on reset indicator. A still further need exists for a technique which may be implemented with a relatively simple circuit. An even further need exists for a technique which is capable of detecting a wide range of power supply thresholds. A still further need exists for a technique which is not highly sensitive to semiconductor process variations and ambient temperature variations.
The present invention provides a circuit for detecting that the level of a power supply is at a low level. Additionally, the circuit also functions as a power on reset indicator. The present invention also provides for a technique which may be implemented with a relatively simple circuit. A circuit of the present invention is capable of detecting a wide range of power supply thresholds. Additionally, the technique is not highly sensitive to semiconductor process variations and ambient temperature variations.
A circuit for power supply detection and power on reset is disclosed. In one embodiment, the circuit comprises two separate component groups for producing separate currents which vary in response to the level of a voltage source. For example, each may have a non-linear DC relationship between the power supply and a current of the respective component group. Furthermore, the DC voltage to current relationship of two groups is different. The component groups may be, for example, current sources or sinks. The circuit has an output with two modes. The two component groups are operable to force the output into one mode when the voltage source is above a threshold and a second mode when the voltage source is below the threshold. Therefore, the circuit is operable to detect when the supply voltage has dropped below a threshold and to also serve as a power on reset. However, the circuit does not rely on the threshold voltage of a transistor to trigger the output. Consequently, it provides for better tolerance than conventional voltage divider circuits, as it is not highly sensitive to semiconductor processing and ambient temperature variations.
These and other advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.